1. Field of the Invention
The present invention relates to a process for stabilizing a hydrocarbonaceous fiber material, especially wood pulp, by an oxidizing alkaline treatment.
2. Description of the Prior Art
The use of oxygen in bleaching cellulose and in delignification processes in general has become an object of growing interest all over the world in recent years. The oxygen-alkali bleaching has considerable advantages when compared with the conventional processes using chlorine chemicals, especially because the enviornmental pollution caused by the dissolved organic material is easier to prevent. Nowadays a considerable part of the waste water load of the factories producing bleached cellulose is produced in connection with bleaching, because usually the organic material present in the waste liquors and which contains, among other things, chlorinated lignin, cannot be eliminated at a reasonable expense. But, the waste liquor from oxygen-alkali bleaching can be concentrated and burned by conventional methods, and the remaining chemicals (alkali) can be recovered and reused.
An important progress took place in oxygen-alkali bleaching technology in the late 1960's after it had been noted that magnesium salts prevent the oxidizing alkaline depolymerization of carbohydrates (French Pat. No. 1,387,853 (1964)). When magnesium carbonate, for example, is added to wood pulp after digestion, the pulp can be treated with oxygen and alkali without causing the cellulose to depolymerize too much in this treatment and thereby worsening the mechanical properties of the obtained fiber too much. It was later suggested that it is advantageous to add magnesium in the form of salts which form complexes with it (Swedish Pat. No. 314,531 (1970)).
Even though the use of magnesium salts has had a great effect on the progress of the oxygen-alkali bleaching process towards the stage of technological realization, it is not yet possible to use solely oxygen in bleaching when normal fully bleached paper pulp is concerned. When the delignification with oxygen is carried too far, the mechanical properties of the fiber suffer greatly even though the above magnesium salts are used as inhibitors in the bleaching. According to current technology, possibly about half of the lignin can be removed with oxygen when a normal chemical sulphate pulp is concerned (Tappi 54 (1971) 1893). In order to achieve a complete delignification, other bleaching chemicals must also be used, usually chlorine and oxidizers based on it. It is clear that the advantages of oxygen-alkali bleaching would become more obvious if the depolymerization of the carbohydrates could be more effectively prevented, in which case the use of chlorine and chlorine chemicals could be reduced or possibly eliminated.
On the basis of the above, systematic experiments have been carried out with the goal of discovering new agents which would work as inhibitors with maximum possible effectiveness in oxygen-alkali bleaching. Cerium and uranium salts and silver proved quite effective among the approximately twenty inorganic compounds tested but their practical application will obviously not be feasible owing to their high price. However, experiments with these materials elucidated the nature of the depolymerization mechanism of carbohydrates, and further investigations were concentrated on organic compounds of a certain type.